Add DNSWATCHER_TARGETS env var that accepts a comma-separated list of
DNS names and automatically classifies them as apex domains or hostnames
using the Public Suffix List (golang.org/x/net/publicsuffix).
- ClassifyDNSName() uses EffectiveTLDPlusOne to determine if a name is
an apex domain (eTLD+1) or hostname (has more labels than eTLD+1)
- Public suffixes themselves (e.g. co.uk) are rejected with an error
- DNSWATCHER_DOMAINS and DNSWATCHER_HOSTNAMES are preserved for
backwards compatibility but logged as deprecated
- When both TARGETS and legacy vars are set, results are merged with
deduplication
Closes#10